To increase
contrast, grab the
tonal curve at a darker tone interval and drag it slightly down. Then grab the
tonal curve at a lighter tone interval and drag it slightly up.
This forms a characteristic "S" curve. You form the "S"
between the two points between which the contrast is to be increased. Illustration

To decrease
contrast, grab the
tonal curve at a darker tone interval and drag it slightly up. Then grab the
tonal curve at a lighter tone interval and drag it slightly down.
This forms a reverse "S" curve. Illustration

The end-points are absolute, 0 and 255, so the
contrast between black and white cannot be increased. (In this tutorial, I
imply two types of contrast: local and global. Local for contrast between two
tones. For an entire image's contrast, since the end-points are fixed, I
look at the histogram's standard deviation as an indicator.) We can, however,
increase the contrast between two points if one is not lying on an extreme.
Increasing the slope of the curve* between the 2
points to more than 45º has the effect of spreading an input tones over a wider
interval of the output line.

When I say slight, I mean slight. To
increase contrast in one range of tones means to decrease contrast in another. If
you go too far, tones in the increased contrast area will start to lose their
continuity and posterize (fewer tones are distributed into a larger number
space), and tones in the decreased area will begin to merge (more tones are
squeezed into a few values). We're restricted to working within a very small
interval of 256 numbers--a zero-sum game. We can perform adjustments on an
image, not miracles. Even the most expert scan cannot compensate for a lack of
tonal range in the source image.

Keep in mind that each pixel in a color image is a
composite of red, green, and blue values. When an RGB curve is applied to
an image, the scanner software or image editing program applies the curve to the
underlying components separately.

Of course, when we modify a curve we don't grab just
any point, we manipulate the curve with specific tonal areas in mind.

Place the cursor (the cross-hairs) on a
point within the area of interest.

The value of the pixel is displayed in the
input field (172) and a knot at that value appears on the curve.

If, for example, you want to lighten
that area, you would grab the curve at that point of the knot and drag
it upwards until it's at the level of the desired output value.Setting gamma

Changes in contrast requires that two points be
identified. An increase in contrast is achieved by moving the two points
so that the slope, or angle, of a line drawn through them is increased. A
decrease in contrast is achieved by moving the two points so that the slope, or
angle, of a line drawn through them is decreased.

With practice, knowing where to grab the tone curve
becomes almost a natural reflex.

Ameliorating
the Loss of Tonal Separation

There are three methods to address the distortions
caused by the application of tone curves:

Scan at double the
resolution, apply curves, and down sample to the lower resolution
This method relies on the down sampling process to average out errors.
This is expensive in storage and CPU resources.

Scan at a greater bit-depth
apply curves and down sample to 8-bits This is the most common
method employed to minimize the effects of tone shifts due to curves.
The Nikon LS-2000 has a provision to scan at 12-bits. This means that
instead of a tonal range of 256 tones, there are 4096, a 16 times finer
tonal resolution. The cost is a doubling of file size. Extra bit
scanning should be considered in the critical scanning of color negatives: A
Brief Note on Scanning Color Negative Film.

Do nothing
Some applications simply do not require very high image quality. Tonal
distortions would not be visible or of little concern to the end-user.
This is particularly true of images destined for Web page display, which
benefit more from an adroit application of an unsharp mask.

That's the theory and received wisdom. In
practice, it's rare that methods 1 and 2, although resulting in better looking
histograms, ever result in visually improved images. The one possible
exception arises when scanning a transparency with important shadow detail or a
transparency that is underexposed. If the source image is properly exposed
and won't undergo extensive tonal modification, 8+-bit scanning is a bit of a
waste. In any case, if you're determined to scan at greater bit depths be
sure to use multi-sampling. It makes little sense to attempt to obtain the
benefits of the precision of 8+-bit depth scanning without the increased accuracy
of multi-sampling.

Tone
Control Sliders

You might wonder what the role of the sliders might
be in the method I have outlined. Simply speaking, none. I hate to
sound preachy and dogmatic, but if there's anything I feel strongly about, it's
that one should not use the sliders to adjust contrast, brightness, or color
balance in creating a final scan. This is not especially because in
NikonScan these adjustments are not reflected in the histogram, thus giving you
no objective feedback, but because the effects of the sliders are so crude and
thus work against the objectives of proper tonal correction. This may be
demonstrated by analyzing the sliders as tone curves.